1. Field of the Invention
The present invention relates to heat exchangers and in particular to the transfer of heat from one liquid, such as a slurry, to another.
2. The Prior Art
A heat exchanger is a device used to transfer heat from one medium to another. In industries such as the mining industry there are many processes that require heating a mineral-ore slurry. A slurry is a suspension of solid particles in a fluid. Slurries contain solid particles that have a tendency to settle. Some slurries also have a tendency to create scale. Both of these issues complicate performing heat-exchange processes, due to the need to periodically clean heat-transfer and other apparatus used in slurry processing.
The high cost of energy makes heat exchangers crucial to the feasibility of these processes. Currently there are no heat exchangers on the market that meet this need. As a result, when it is necessary to heat a mineral-ore slurry in a countercurrent manner (by cooling another slurry passing in the opposite direction), very complex systems are used, such as contact heat exchangers in which steam is evolved from one slurry and absorbed into the other slurry in an adjacent manifold. This is the typical type of exchanger used in Bayer Process plants for producing alumina from Bauxite ore.
Slurries have been run through existing heat exchanger configurations such as spiral or plate heat exchangers. A spiral exchanger includes a pair of flat surfaces that are coiled to form two channels in a counter current arrangement with each channel having a long curved path. A plate exchanger is composed of multiple, thin, slightly separated plates that have very large surface areas and fluid passages for heat transfer.
Although spiral and plate exchangers are promoted as being able to handle slurries, they employ fluid passages having physical dimensions that are typically not conducive to maintaining a good suspension of solids in the slurry. Spiral and plate exchangers do not have easily accessible passages and in some cases have no access at all, leading to high maintenance costs. Spiral and plate exchangers can be used in some slurry applications, but in fact they can be used only for relatively simple and dilute slurries in which the slurry particles stay easily suspended in the liquid.
Shell-and-tube exchangers are currently used in some slurry applications as well. A shell and tube exchanger consists of a series of tubes running through a shell and containing a medium to be either heated or cooled. The shell (or larger tube) contains a second flowing medium which either provides or absorbs the heat as required.
Currently it is possible to heat a slurry in the tube side of a shell-and-tube exchanger in which the shell contains a non slurry (liquid or steam), but it is not possible to transfer heat from a slurry to a slurry, because the slurry cannot be run in the shell side, where large particles will settle out, causing fouling and eventually blockage.
In the 1990's, several plants that processed nickel ore were installed in Australia, all using high temperature autoclaves. Extensive research was done for the design of these plants in order to select an effective heat exchanger. However, the best system which was found was a system in which steam was extracted by a slight vacuum from the slurry and then recondensed in the shell of a shell and tube exchanger to heat the slurry passing in the opposite direction. Although these Nickel plants involved a combined capital investment of over US $1 Billion, the designers were not able to find a better way to transfer heat because there existed no design for a simple countercurrent exchanger which could transmit heat from one slurry to another. These complex heat exchange systems recycle only about 70 percent of the heat, representing a missed opportunity to significantly reduce operating costs.
Graphite block heat exchangers are also known in the art. In these exchangers graphite blocks are drilled with several closely spaced parallel holes for carrying the solution to be heated (or cooled), and other holes are drilled at right angles to them to carry the heating (or cooling) fluid. Such exchangers are widely used for heating and cooling acids. However they are limited in their usefulness for several reasons: graphite is soft and cannot be used for abrasive slurries; graphite can be oxidized and so is not chemically stable for some applications; graphite is brittle and has low strength, so the pressure at which these exchangers can operate is limited (high pressure causes cracks to form and propagate from tube to tube). Also, because of the brittleness of the graphite it is difficult to establish a tube header on the ends so that a simple straight-flowpath parallel tube arrangement is not possible. To avoid this problem, graphite exchangers are designed with a cross-flow tube arrangement but this is not nearly as effective as a parallel flow arrangement.
Similar exchangers using other materials substituted for graphite have not been encountered. The main reason would seem to be that drilling or otherwise machining blocks formed from materials such as metal is expensive. An example of such an exchanger is disclosed in U.S. Pat. No. 1,799,626, disclosing tubes cast in a metal block. The tubing would not be effective in accomplishing countercurrent heating/cooling. Similarly, as shown in U.S. Pat. No. 4,711,298, ceramic block exchangers (similar to the graphite block exchangers) are known, but ceramic material also has the brittle qualities of graphite so the tubing arrangements are not simple enough for slurries. The inability of the existing technology to serve the needs of industries such as the mining industry is confirmed by the fact that a need exists, but there are no simple heat exchangers to serve that need.